Laser mask and crystallization method using the same

ABSTRACT

A crystallization method using a mask includes providing a substrate having a semiconductor layer; positioning a mask over the substrate, the mask having first, second and third blocks, each block having a periodic pattern including a plurality of transmitting regions and a blocking region, the periodic pattern of the first block having a first position, the periodic pattern of the second block having a second position, the periodic pattern of the third block having a third position, the first, second and third positions being different from each other; and crystallizing the semiconductor layer by irradiating a laser beam through the mask.

This application claims the benefit of Korean Patent Application No.2003-99387, filed on Dec. 29, 2003, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser mask and a crystallizationmethod using the same, and more particularly, to a laser mask and acrystallization method using the same that can improve thecrystallization characteristics of silicon thin film.

2. Discussion of the Related Art

Recently, due to the needs for information displays, especially forportable information displays, thin film type flat panel display (FPD)devices have been actively being researched and commercialized such thatthe cathode ray tubes (CRT) are being replaced. Of these flat paneldisplay devices, a liquid crystal display (LCD) device displays imagesusing an optical anisotropy of liquid crystal. An LCD device can be usedfor notebook computers, desktop monitors, and other display devices, dueto its excellent resolution, color rendering capability and picturequality.

An active matrix (AM) driving method, a typical driving method used inthe LCD device, drives each of the pixels of the LCD device using anamorphous silicon thin film transistor (a-Si TFT) as a switching device.The a-Si TFT technique was described by English LeComber et al. in 1979,and was commercialized as a three-inch liquid crystal portabletelevision in 1986. Recently, a TFT-LCD device with a display area ofmore than 50 inches has been developed. However, the field effectmobility of the a-Si TFT is about 1 cm²/Vsec, which prevents its use inperipheral circuits that apply signals to the pixels, because theperipheral circuits generally operate at more than 1 MHz. Accordingly,researches for simultaneously forming switching transistors in a pixelregion and peripheral circuits in a driving circuit region on a glasssubstrate using a polycrystalline silicon (poly-Si) TFT having a fieldeffect mobility greater than that of the a-Si TFT have been activelypursued.

The poly-Si TFT has been applied to small flat panel displays, such asthe eyepiece of camcorders, since an LCD color television was developedin 1982. Such a TFT has low photosensitivity and high field effectmobility, and it can be directly fabricated on a substrate to formdriving circuits. Increased mobility can increase the operationfrequency of the driving circuits. The frequency capability of thedriving circuits determines the number of the pixels that can be drivenwhile maintaining adequate display capability. More specifically, theincreased frequency decreases the charging time of a signal applied to apixel such that distortion of the signal is decreased and the picturequality increases. Compared to the a-Si TFT, which has a high drivingvoltage of about 25V, the poly-Si TFT, which has a driving voltage ofunder 10V, consumes less power.

The poly-Si TFT can be fabricated by directly depositing apolycrystalline silicon thin film on a substrate or by depositing anamorphous silicon thin film that is then crystallized by a thermalprocess. To use a cheap glass as a substrate, low temperature processesare required, and, to use the poly-Si TFT for driving circuits, a methodfor increasing the field effect mobility is required. In general,thermal processing methods for crystallizing an amorphous silicon thinfilm are the solid phase crystallization (SPC) method and the excimerlaser annealing (ELA) method.

The SPC method forms a polycrystalline silicon thin film at a lowtemperature of approximately 600° C. In this method, a polycrystallinesilicon thin film is formed by depositing an amorphous silicon thin filmon a glass substrate having a low melting point and then by performing aslow heating process at approximately 600° C. for up to tens of hours. Apolycrystalline silicon thin film obtained by the SPC method hascomparatively large-size grains of about several mm (micrometers).However, there are many defects in the grains. Although not as bad asgrain boundaries in a poly-Si TFT, these defects affect negatively onthe performance of a poly-Si TFT.

The excimer laser annealing method is a typical method of fabricating apoly-Si TFT at a low temperature. The excimer laser crystallizes anamorphous silicon thin film by irradiating a high energy laser beam ontothe amorphous silicon thin film for a time of ten nanoseconds. In thismethod, the amorphous silicon is melted and crystallized in a very shorttime, so that the glass substrate is not damaged. A polycrystallinesilicon thin film fabricated by the excimer laser method also hasexcellent electrical characteristics, compared to a poly-Si thin filmfabricated by a general thermal processing method. For example, a fieldeffect mobility of a poly-Si TFT fabricated by the excimer laser methodis more than 100 cm²/Vsec, whereas a field effect mobility of an a-SiTFT is 0.1˜0.2 cm²/Vsec and a field effect mobility of a poly-Si TFTfabricated by a general thermal processing method is 10˜20 cm²/Vsec(IEEE Trans. Electron Devices, vol. 36, no. 12, p.2868, 1989).

A crystallization method using a laser will now be described in detail.FIG. 1 is a graph illustrating a relationship between a grain size of apolycrystalline silicon thin film and an energy density of a laser usedto form the polycrystalline silicon thin film.

As shown in FIG. 1, in the first and second regions I and II, as theenergy density increases, the grain size of the polycrystalline siliconthin film increases, as discussed in IEEE Electron Device Letters,DEL-7, 276, 1986. However, in the third region III, when the energydensity becomes higher than a specific energy density Ec, the grain sizeof the crystallized polycrystalline silicon thin film decreasesdrastically. That is, according to the graph shown in FIG. 1, thecrystallization mechanism for the silicon thin film becomes differentwhen the energy density is higher than a specific energy density Ec.

FIGS. 2A to 2C, 3A to 3C and 4A to 4C are sectional views illustratingsilicon crystallization mechanisms according to the laser energydensities of FIG. 1. That is, they illustrate sequential crystallizationprocess according to each laser energy density. A crystallizationmechanism of amorphous silicon by a laser annealing is influenced bymany factors, such as laser irradiation conditions including laserenergy density, irradiation pressure, substrate temperature, andphysical/geometrical characteristics including absorption coefficient,thermal conductivity, mass, impurity containing degree and amorphoussilicon layer thickness.

First, as shown in FIGS. 2A to 2C, the first region (I) of FIG. 1 is apartial melting region, and an amorphous silicon thin film 12 iscrystallized only up to the dotted line and a size of a grain G1 formedat this time is about hundreds Å. When a laser beam is irradiated on theamorphous silicon thin film 12 on a substrate 10 where a buffer layer 11is formed, the amorphous silicon thin film 12 is melted. At this time,because strong laser energy is irradiated directly at a surface of theamorphous silicon thin film 12 and relatively weak laser energy isirradiated at a lower portion of the amorphous silicon thin film 12, acertain portion of the amorphous silicon thin film 12 is melted. As aresult, crystallization is partially performed.

Typically, in the laser crystallization method, crystals grow throughthe processes of primary melting in which a surface layer of anamorphous silicon thin film is melted by a laser irradiation, secondarymelting in which a lower portion of the amorphous silicon thin film ismelted by the latent heat generated during the solidification of themelted silicon, and the solidification of the lower layer. These crystalgrowth processes will be explained in more detail.

An amorphous silicon thin film on which a laser beam is irradiated has amelting temperature of more than 1000° C. and primarily melts into aliquid state. Because there is a great temperature difference betweenthe surface melted layer and the lower silicon and substrate, thesurface melted layer cools fast until solid phase nucleation andsolidification are achieved. The surface layer remains melted until thesolid phase nucleation and solidification are completed. The meltingstate lasts for a long time when the laser energy density is high orthermal emission to the outside is low. Because the surface layer meltsat a lower temperature than the melting temperature of 1400° C. forcrystalline silicon, the surface layer cools and maintains asuper-cooled state where the temperature is lower than the phasetransition temperature.

The greater the super-cooling state is, that is, the lower the meltingtemperature of the thin film or the faster the cooling speed is, thegreater the nucleation rate is at the time of the solidification suchthat fine crystals grow during the solidification. When thesolidification starts as the melted surface layer cools, crystals growin an upward direction from a crystal nucleus. At this time, latent heatis generated during the phase transition of the melted surface layerfrom liquid state to solid state, and thus the secondarily meltingbegins where the lower amorphous silicon thin film melts. Then, thesolidification of the lower amorphous silicon thin film occurs. At thistime, the nucleus generation rate of the lower second melted layerincreases, because the lower amorphous silicon thin film is moresuper-cooled than the first melted layer. Thus, the crystal sizeresulting from the second melted layer is smaller. Accordingly, thecooling speed of the solidification has to be reduced to improve thecrystalline characteristics. Cooling speed can be reduced by restrainingabsorbed laser energy from being emitted to the outside. Examples of therestraining method are heating the substrate, double beam irradiation,or inserting a buffer insulating layer between the substrate and theamorphous silicon layer.

FIGS. 3A to 3C are sectional views illustrating the siliconcrystallization mechanism of the second region (II) of FIG. 1, in whichthe second region (II) represents a near-completely crystallized region.

Referring to FIGS. 3A to 3C, a polycrystalline silicon thin film hasrelatively large grains 30A-30C of about 3000 to 4000 Å formed down tothe interface of the lower buffer layer 11. When a nearly completemelting energy, not a complete melting energy, is irradiated on theamorphous silicon thin film 12, almost all of the amorphous silicon thinfilm 12 down close to the buffer layer 11 melts. At this time, solidseeds 35 that have not been melted at the interface between the meltedsilicon thin film 12′ and the buffer layer 11 work as a crystallizationnucleus to induce side growth, thereby forming the relatively largegrains 30A-30C (J. Appl. Phys. 82, 4086). However, because thiscrystallization only occurs when the laser energy is such that the solidseeds 35 remain on the interface with the buffer layer 11, the processmargin is very limited. In addition, because the solid seeds 35 aregenerated non-uniformly the crystallized grains 30A-30C of thepolycrystalline silicon thin film have different crystallizationdirections, thereby resulting in non-uniform crystallizationcharacteristics.

FIGS. 4A to 4C are sectional views illustrating the siliconcrystallization mechanism of the third region (III) of FIG. 1corresponding to a completely crystallized region.

Referring to FIGS. 4A to 4C, very small grains 30 are irregularly formedwith a energy density corresponding to the third region (III). When thelaser energy density becomes higher than a specific energy density Ec,sufficient energy is applied enough to completely melt the amorphoussilicon thin film 12, leaving no solid seeds that may be grown tograins. Thereafter, the silicon thin film 12′ which has been melted uponreceiving the laser beam of the strong energy undergoes a rapid coolingprocess, which generates a plurality of uniform nuclei 35 and thus finegrains 30.

Meanwhile, an excimer laser annealing method employing a pulse-typelaser is typically used for the laser crystallization, and a sequentiallateral solidification (SLS) method, which shows remarkable improvementof crystallization characteristics by growing grains in a horizontaldirection, has recently been proposed and studied widely.

The sequential lateral solidification (SLS) utilizes the fact thatgrains grow laterally from an interface between liquid phase silicon andsolid phase silicon (Robert S. Sposilli, M. A. Crowder, and James S. Im,Mat. Res. Soc. Symp. Proc. Vol. 452, 956 to 957, 1997). In this method,grains grow laterally with a predetermined length by controlling thelaser energy density and irradiation range of a laser beam, therebyincreasing the size of silicon grains.

This SLS is one example of lateral solidification (LS), and thecrystallization mechanism with respect to the LS will now be describedwith reference to the accompanying drawings. FIGS. 5A to 5C aresectional views illustrating a sequential crystallization processaccording to a related art.

Referring to FIG. 5A, when a laser having an energy density in the thirdregion (III) of FIG. 1, the energy density capable of completely meltingan amorphous silicon thin film 112, is irradiated onto a portion of anamorphous silicon thin film 112, the portion of the amorphous siliconfilm completely melts. A patterned mask can be employed to form a laserirradiated region and a laser non-irradiated region. At this time, asshown in FIGS. 5B and 5C, because the laser has sufficient energy, theamorphous silicon thin film 112 irradiated by the laser can becompletely melted. However, the laser beam is irradiated with certainintervals on the amorphous silicon thin film 112, crystals grow from theinterface between the silicon thin film 112 of the laser non-irradiatedregion (solid phase) and the melted silicon thin film 112′ (liquidphase).

Thus, the interface provides nuclei for this crystallization. In otherwords, immediately after the laser beam is irradiated, the meltedsilicon thin film 112′ cools from the left/right surfaces, theinterfaces of the laser non-irradiated region. This is because the solidphase amorphous silicon thin film 112 has higher heat conductivity thanthe buffer layer 111 or the glass substrate 110 below the silicon thinfilms 112 and 112′. Accordingly, the melted silicon thin film 112′ firstreaches a nucleus formation temperature at the interface between thehorizontal solid phase and the liquid phase, rather than at the centralportion, forming a crystal nucleus at the corresponding portion. Afterthe crystal nucleus is formed, grains 130A and 130B horizontally growfrom a low temperature side to a high temperature side, that is, fromthe interface to the central portion. Due to the lateralcrystallization, large-size grains 130A and 130B can be formed, andbecause the process is performed with the energy density of the thirdregion, the process margin is not limited, compared to other regions.However, the SLS has the following problems.

That is, the crystallization is performed by infinitesimally andrepeatedly moving the mask or the stage in order to increase the size ofthe grains. As a result, the process time for crystallizing a large-sizeamorphous silicon thin film is lengthened and the process yield becomeslow.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a laser mask and acrystallization method using the same that substantially obviates one ormore of the problems due to limitations and disadvantages of the relatedart.

An advantage of the present invention is to provide a laser mask and acrystallization method using the same that can improve thecrystallization characteristics of silicon thin film.

Still another advantage of the present invention is to provide a liquidcrystal display device including a silicon thin film having improvedcrystallization characteristics fabricated by the crystallization methoddescribed herein.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a lasermask includes first, second and third blocks, each block having aperiodic pattern including a plurality of transmitting regions and ablocking region, the periodic pattern of the first block having a firstposition, the periodic pattern of the second block having a secondposition, the periodic pattern of the third block having a thirdposition, the first, second and third positions being different fromeach other.

In another aspect of the present invention, a crystallization methodusing a mask includes providing a substrate having a semiconductorlayer; positioning a mask over the substrate, the mask having first,second and third blocks, each block having a periodic pattern includinga plurality of transmitting regions and a blocking region, the periodicpattern of the first block having a first position, the periodic patternof the second block having a second position, the periodic pattern ofthe third block having a third position, the first, second and thirdpositions being different from each other; and crystallizing thesemiconductor layer by irradiating a laser beam through the mask.

In another aspect of the present invention, a display device includes agate line and a data line crossing each other to form a pixel; a thinfilm transistor (TFT) near the crossing, the TFT including apolycrystalline silicon layer, wherein the polycrystalline silicon layerincludes a plurality of circular crystals, and the three adjacentcircular crystals form one equilateral triangle, and six of theequilateral triangles form a regular hexagon.

In yet another aspect of the present invention, a display deviceincludes a gate line and a data line crossing each other to form apixel; a thin film transistor (TFT) near the crossing, the TFT includinga polycrystalline silicon layer, wherein the polycrystalline siliconlayer includes a plurality of crystals having a polygon shape, and thecenters of the three adjacent crystals form one equilateral triangle,and six of the equilateral triangles form a regular hexagon.

In still another aspect of the present invention, a method forfabricating a display device includes forming a plurality of gate linesand data lines on a substrate, the gate and data lines crossing eachother to define pixels; and forming a thin film transistor (TFT) neareach crossing in the pixel, this step further including forming asemiconductor layer on the substrate; positioning a mask over thesubstrate, the mask having first, second and third blocks, each blockhaving a periodic pattern including a plurality of transmitting regionsand a blocking region, the periodic pattern of the first block having afirst position, the periodic pattern of the second block having a secondposition, the periodic pattern of the third block having a thirdposition, the first, second and third positions being different fromeach other; crystallizing the semiconductor layer by irradiating a laserbeam through the mask.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a graph illustrating a relationship between a grain size of apolycrystalline silicon thin film and an energy density of a laser usedto form the polycrystalline silicon thin film.

FIGS. 2 to 4 are sectional views illustrating silicon crystallizationmechanisms according to the laser energy densities of FIG. 1;

FIG. 5A to 5C are sectional views illustrating a sequentialcrystallization process according to a related art.

FIG, 6A is a plan view illustrating an example of a laser mask used fora sequential lateral solidification (SLS);

FIG. 6B is a plan view illustrating a silicon thin film crystallized bythe mask of FIG. 6A;

FIG. 7 is an enlarged plan view illustrating the portion ‘E’ of thecrystallized silicon thin film of FIG. 6B;

FIGS. 8A to 8C are plan views illustrating a sequential process forcrystallizing a silicon thin film using the mask of FIG. 6A;

FIG. 9 illustrates another example of a laser mask used for the SLS;

FIG. 10 illustrates a method for constructing periodic patterns in alaser mask according to the present invention;

FIG. 11 illustrates a size of a transmitting region of the laser mask ofFIG. 10;

FIG. 12 illustrates a method for constructing mask patterns divided intothree blocks for the laser mask of FIG. 10;

FIGS. 13A to 13C illustrate three blocks of a laser mask constructedaccording to the method described in FIG. 12.

FIGS. 14A to 14C illustrate a process for crystallizing a silicon thinfilm using the laser mask of FIGS. 13A to 13C.

FIG. 15A illustrates a method for constructing a laser mask according toa first embodiment of the present invention.

FIG. 15B illustrates an example of a laser mask fabricated by thepattern constructing method with reference to FIG. 15A.

FIGS. 16A to 16H illustrates a sequential process for crystallizing asilicon thin film using the laser mask shown in FIG. 15B.

FIG. 17 illustrates a method for constructing periodic patterns in alaser mask according to a second embodiment of the present invention.

FIG. 18A illustrates a method for constructing a laser mask inaccordance with the second embodiment of the present invention.

FIG. 18B illustrates an example of a laser mask fabricated by thepattern constructing method described with reference to FIG. 18A.

FIGS. 19A to 19G illustrates a sequential process for crystallizing asilicon thin film using the laser mask shown in FIG. 18B.

FIG. 20 is a plan view illustrating a structure of a liquid crystaldisplay panel, in which a driving circuit is integrated with the arraysubstrate of the LCD panel.

FIG. 21 illustrates an example of an LCD device fabricated using asilicon thin film crystallized by a crystallization method in accordancewith the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 6A is a plan view illustrating an example of a laser mask used fora sequential lateral solidification (SLS), which is designed to shortencrystallization time compared to the related art. Referring to FIG. 6A,a laser mask 270 includes a slit type pattern 275 having a rectangulartransmitting region 273 with a predetermined width and length. The lasermask 270 includes two rectangular transmitting regions 273 fortransmitting light and blocking region 274 for blocking light. A laserbeam transmitted through the transmitting regions 273 of the slit 275crystallizes a silicon thin film according to the shape (e.g.,rectangular shape) of the transmitting regions 273.

Referring to FIG. 6B, however, an edge portion (E) of the crystallizedsilicon thin film has a round shape different from the mask pattern (theslit 275), due to the diffraction of the laser beam. This will now bedescribed in detail. For reference, the dotted line at the edge portion(E) of the crystallized silicon thin film shown in FIG. 6B illustratesthe shape of the slit 275 of the mask 270 used for the crystallization.

FIG. 7 is an enlarged plan view illustrating the portion ‘E’ of thecrystallized silicon thin film of FIG. 6B. As shown in FIG. 7, a region‘A’, the center of the edge portion (E) has a similar crystallizationpattern to the slit 275, because the laser beam having an energy densitysufficient to completely melt the silicon film is irradiated. However,the laser beam is diffracted at a region ‘B’, corners of the edgeportion (E) of the slit 275. Thus, the laser beam cannot have an energydensity sufficient to completely melt the silicon thin film. As aresult, the edge portion (E) becomes to have a convex or round shape. Inother words, because the grains in the edge portion (E) of thecrystallized silicon thin film having the round shape is grown fromnuclei formed near the amorphous silicon thin film (solid phase) at theinterface of the melted amorphous silicon, a second grain 230B grows ina direction different from a first grain 230A. That is, the second grain230B has different crystallization characteristics from the first grain230A, and as a result, a discontinuous region exists in the crystallizedsilicon thin film. At this time, because the discontinuous region havinga width (W), the convex edge portion (E) of the crystallized siliconthin film, has different crystallization characteristics, the width (W)of the discontinuous region needs to be reduced in order to apply thesilicon thin film to an LCD device.

A crystallization process for crystallizing the silicon thin film usingthe above-described mask will now be described. FIGS. 8A to 8C are planviews illustrating a sequential process for crystallizing a silicon thinfilm using the mask of FIG. 6A.

First, as shown in FIG. 8A, the mask 270 of FIG. 6A is positioned on asubstrate 210 to which a first laser beam is irradiated to crystallizean amorphous silicon thin film 212 formed on the substrate 210. At thistime, the crystallized region corresponds to the transmitting region 273of the mask 270, and when the mask 270 has two transmitting regions, thecrystallized region has two crystallized regions having a predeterminedlength in a horizontal direction. In other words, when the first laserbeam is irradiated on the surface of the substrate 210 using the mask270 including the rectangular slit 275, the silicon thin film, to whichthe first laser beam has been irradiated through the slit 275, has afirst grain 230A grown laterally (vertically in FIG. 8A) from nucleiformed near the amorphous silicon thin film 212 (solid phase) positionedat the upper and lower boundary surfaces. At this time, as mentionedabove, the edge portions of the crystallized silicon thin film 212′ havea round shape different from the mask pattern, the shape of the slit275, due to the diffraction of the laser beam, and at the rounded edgeportions (E), a second grain 230B grows from nuclei formed near theamorphous silicon thin film 212 (solid phase) positioned at the boundarysurface of the melt amorphous silicon in a direction different from thefirst grain 230A. That is, the second grain 230B has crystallizationcharacteristics different from the first grain 230A, and a discontinuousregion exists in the crystallized silicon thin film.

After the first crystallization is completed, the stage (not shown) orthe mask 270 placed over the substrate 210 is moved by a short distancenot greater than the horizontal length of the pattern of the mask 270(the width of the slit 275), and then a second laser beam is irradiatedto proceed with the crystallization process in the direction of ‘X’axis. For example, after the stage is moved in the direction of ‘−X’axis to overlap the discontinuous region 280 of the slitpattern-crystallized silicon thin film 212′, the second laser beam isthen irradiated on the surface of the substrate 210.

Then, as shown in FIG. 8B, the second crystallized pattern 212″ havingthe same pattern as the silicon thin film 212′ crystallized by the firstcrystallization is formed in the direction of ‘X’ axis, whileoverlapping the discontinuous region 280 of the first crystallizedsilicon thin film 212′. Thereafter, when a third laser beam isirradiated on the surface of the substrate 210 in the same manner asdescribed with respect to the first laser beam, the thirdcrystallization pattern 212′″ having the same pattern as the siliconthin film 212″ crystallized by the second crystallization is formed,while overlapping the discontinuous region 280 of the secondcrystallized silicon thin film 212″. At this time, the wider thediscontinuous region 280, the wider the overlap region of the laser beamfor the next shot, which increases the overall process time. Thediscontinuous regions 280 of the crystallized silicon thin films 212′,212″ and 212′″ have different crystallization characteristics, and inthis respect, because the silicon thin film 212 around the discontinuousregions 280 remains in an amorphous state, without being crystallized,the next shot of the laser beam is required to overlap thesediscontinuous regions 280.

After the crystallization process in the direction of ‘X’ axis iscompleted, the mask 270 or the stage is moved by a predetermineddistance in the direction of ‘Y’ axis (in case of moving the stage, itis moved in the direction of ‘−Y’ axis). And then, as shown in FIG. 8C,the laser irradiation process is performed again in the direction of ‘X’axis, starting from the point where the first crystallization processwas finished.

When the above-described crystallization process is repeatedlyperformed, a problem arises in that the polycrystalline silicon thinfilm has a plurality of first regions (P1) having normal grains and aplurality of second regions (P2) having the discontinuous regions, whichhave different crystallization characteristics and are located betweenthe first regions P1. That is, when an LCD device is fabricated byincorporating such a silicon thin film having these discontinuousregions, the LCD device suffers from uneven characteristics, and thus,the quality of the LCD device becomes degraded. In addition, because thesilicon thin film around the discontinuous regions remains in anamorphous silicon state, rather than having been crystallized, the nextshot of the laser beam is required to overlap these discontinuousregions 280. These overlap regions (namely, X-overlap regions) in whichthe discontinuous regions overlap each other produce a shot mark. Theshot mark decreases picture quality and produces non-uniform devicecharacteristics, when it is applied to an LCD device or an organic lightemitting diode.

Meanwhile, although not explained in the above crystallization process,the grains can be grown in the direction of ‘Y’ axis and the maskoverlaps in the direction of ‘Y’ axis in order to increase the size ofthe grains, and then, the crystallization can be repeatedly performed.In this case, however, the shot mark may be produced in the overlapregions (namely, Y-overlap regions) in the direction of ‘Y’ axis.

The shot mark is also a critical matter when a laser mask 370 of asingle scan method is employed, which is shown in FIG. 9, as well aswhen the above-described transition method (multiple scan method) isemployed. That is, the shot mark problem needs to be solved in everycrystallization method where the laser beam overlaps. Thus, the presentinvention discloses a laser mask and a crystallization method using thesame that do not form such an overlap region in a crystallized siliconfilm. To this end, a laser mask according to the present invention hasperiodic patterns.

A laser mask according to the present invention is divided into threeblocks, each block having a periodic pattern. A laser beam is irradiatedonto a silicon thin film three times, each time using one of the threeblocks. A silicon thin film crystallized by the above mentioned method(three-shot method) has uniform crystallization characteristics withouthaving X-overlap or Y-overlap regions, due to the periodic patterns ofthe mask. The crystallized silicon thin film formed by the periodic maskpattern and the three-shot method has uniform grains, which are grownradially, without having a shot mark, which will now be described indetail.

First, a method for constructing such periodic patterns in a laser maskwill now be described. FIG. 10 illustrates a method for constructingperiodic patterns in a laser mask according to the present invention.The laser mask has three blocks, each block having its own periodicpattern.

Referring to FIG. 10, a laser mask according to the present inventionincludes a plurality of transmitting regions having a circular shape.The laser mask is divided into three blocks to solve the shot markproblem. A transmitting region 475A with a position ‘A’ is formed in afirst block, and either a transmitting region 475B with a position ‘B’or a transmitting region 475C with a position ‘C’ is formed in a secondblock. The positions A, B and C are shown in FIG. 10, and the positionsA, B and C and the relationships among them will be described later indetail. Thus, when the second block has the transmitting region 475B,then the third block has the transmitting region 475C. On the otherhand, when the second block has the transmitting region 475C, then thethird block has the transmitting region 475B. That is, one of the threeblocks of the laser mask has one of the transmitting regions 475A to475C.

Although the laser mask pattern is formed on the basis of thetransmitting region 475C of the position ‘C’, the transmitting region475A of the position ‘A’ or the transmitting region 475B of the position‘B’ can be used as a reference point. When the transmitting region 475Cof the position ‘C’ is used as a reference point, the transmittingregion 475A of the position ‘A’ and the transmitting region 475B of theposition ‘B’ surround the reference point 475C.

When an amorphous silicon thin film is crystallized using the laser maskhaving the three patterns 475A to 475C, the neighboring three patterns475A to 475C form one equilateral triangle, and six equilateraltriangles form a regular hexagon, as shown in FIG. 10. In other words,the mask pattern 475C of the position ‘C’ or the mask pattern 475B ofthe position ‘B’, which is formed in the second block, is positioned atthe center of the regular hexagon pattern, and the mask patternsdifferent from the center pattern surround the center of the regularhexagon pattern. In addition, when an amorphous silicon thin film iscrystallized by sequentially applying the three mask patterns 475A to475C, the neighboring three patterns 475A to 475C are positioned at thevertexes of the equilateral triangle.

Meanwhile, the size and intervals of the three periodic patterns 475A to475C should satisfy a certain relationship in order for the laser maskto completely crystallize amorphous silicon by irradiating three times(three-shot) without a shot mark. This will be described as follows.

FIG. 11 illustrates a size of a transmitting region of the laser mask ofFIG. 10, taking an example of the transmitting region of the position‘A’. As shown, assuming that a radius of the transmitting region 475A ofthe position ‘A’ is ‘R’ and a distance between the centers of thetransmitting regions 475A is ‘L’, the radius (R) of the transmittingregion should satisfy equation (1) in order to crystallize the overallregion.${{equation}\quad(1)\text{:}\quad\frac{L}{3}} \leq R < \frac{L}{2}$If the radius (R) of the transmitting regions of the mask patterns (475Ato 475C) is smaller than L/3, the overall region cannot be crystallizedby the three-shot, and if the radius (R) is greater than L/2, then themask patterns (475A to 475C) contact each other.

A laser mask that has the three mask patterns in three blocks will nowbe described in detail. FIG. 12 illustrates a method for constructingthe mask patterns divided into three blocks for the laser mask of FIG.10.

Referring to FIG. 12, mask patterns 575A to 575C are positioned in orderat the corners of the equilateral triangles, which constitute theregular hexagon shown in FIG. 10. Taking an example of the first row,the mask pattern 575C of the position ‘C’ and the mask pattern 575B ofthe position ‘B’ are repeatedly positioned in order in the direction ofthe ‘X’ axis, starting from the mask pattern 575A of the position ‘A’.For the second row, after being moved by a distance corresponding to onehalf of the length (L′) of the side of the equilateral triangle in themask patterns 575A to 575C in the first row, another set of the maskpatterns 575A to 575C is positioned. In other words, in the second row,after being moved by L′/2 in the direction of X axis, the mask pattern575B of the position ‘B’, the mask pattern 575A of the position ‘A’ andthe mask pattern 575C of the position ‘C’ are repeatedly positioned inorder in the direction of X axis. The three mask patterns 575A to 575Cin the second row constitute equilateral triangles together with theneighboring mask patterns 575A to 575C in the first row. The third row(namely, odd number rows) is constructed in the same manner as the firstrow, and the fourth row (namely, even number rows) is constructed in thesame manner as the second row. For the Y-axis direction, the threepatterns in the next row is moved a distance by L/2 (namely, ½ of thedistance (L) between the centers of the mask patterns 575A to 575C) withrespect to the prior row. By dividing the three periodic mask patternsinto three blocks in a laser mask and applying the laser mask to thethree-shot crystallization method, a crystalline silicon thin film canbe obtained without an X-overlap or a Y-overlap. This will now bedescribed.

FIGS. 13A to 13C illustrate three blocks of a laser mask constructedaccording to the method described in FIG. 12. In the laser mask, themask pattern 575C of the position ‘C’ is formed in the second block, andthe mask pattern 575B of the position ‘B’ is formed in the third block.As shown, each block (580′ to 580′″) includes multiple transmittingregions 573A to 573C having a circular shape and blocking regions 574Ato 574C for blocking light. The first block 580′ includes the maskpattern 575A positioned in the first, fourth and seventh rows of FIG.12. The second block 580″ includes the mask patterns 575C positioned inthe third, sixth and ninth rows. The third block 580′″ includes the maskpatterns 575B positioned in the second, fifth and eighth rows. Althoughthe transmitting regions of the mask patterns 575A to 575C have acircular shape in the drawings, they can be also formed to have aregular polygon shape such as regular triangle, square, regular hexagonand regular octagon without being limited thereto. In addition, in thedrawings, although the radius (R) of the circular mask patterns 575A to575C is one third of the distance (L) between the centers of maskpatterns 575A to 575C, it is not limited thereto, so long as therelationship between R and L satisfies equation 1.

FIGS. 14A to 14C illustrate a process for crystallizing a silicon thinfilm using the laser mask of FIGS. 13A to 13C. A silicon thin filmcrystallized by the three-block laser mask having the periodicitydescribed above has uniform crystallization characteristics without ashot mark.

First, as show in FIG. 14A, when a first laser beam is irradiated onto asilicon film 512 on a substrate 510 through the mask pattern 575A of theposition ‘A’ (namely, the transmitting regions 573A of the mask pattern575A) formed in the first block 580′, grains grow toward the centers ofthe circular pattern 573A using the amorphous silicon thin film (solidphase) 512 positioned at the boundary surface as a nucleus, therebyforming first polycrystalline crystals 512′ having a circular shape. Theregions crystallized by this first crystallization correspond to thetransmitting regions 573A of the laser mask. Thus, if there are eighttransmitting regions in the first block of the laser mask, eightpolycrystalline crystals 512′ having a circular shape will be formed inthe silicon thin film 512.

After the first crystallization is completed, a second laser beam isirradiated onto the silicon thin film 512 having the firstpolycrystalline crystals 512′ through the second block 580″ of FIG. 13B.This second crystallization uses the second block 580″, where the maskpattern 575C of the position ‘C’ is formed, without moving the substratein the direction X or Y. As a result, as shown in FIG. 14B, startingfrom the circumferences of the patterns of the first polycrystallinecrystals 512′, grains grow toward the centers of the mask pattern 575Cof the second block 580″ to form the second polycrystalline crystals512″. The second crystallization is such that three of the firstcrystals 512′ surround one of the second crystals 512″, and the secondcrystallization starts from the regions in which the mask pattern 575Cof the position ‘C’ overlaps the three first crystals 512′. As a result,the second crystals 512″ grow toward the centers of the mask pattern575C of the position ‘C’.

Next, a third laser beam is irradiated onto the silicon thin film 512having the first and second polycrystalline crystals 512′ and 512″through the third block 580′″ of FIG. 13C, in which the mask pattern575B of the position ‘B’ are formed. Then, as shown in FIG. 14C,starting from the regions 520″ where the patterns of the second crystals512″ overlap the mask pattern 575B of the position ‘B’, grains growtoward the centers of the mask pattern 575B of the third block 580′″ toform the third polycrystalline crystals 512′″, which completes thecrystallization of the silicon thin film 512.

In this manner, the three-shot method completely crystallizes thesilicon film 512 on the substrate 510 using the laser mask without anX-overlap or a Y-overlap, that is, without a shot mark. As explained,the laser mask has three blocks, each block having a periodic pattern.At this time, the first, second and third crystals 512′, 512″ and 512′″formed by the three-shot method have the same circular shape as the maskpatterns 575A to 575C, and accordingly, the crystallized silicon thinfilm becomes to have uniform grains, which are grown radially.

A laser mask and a process for crystallizing a large-size silicon thinfilm using the same according to the present invention will now bedescribed. FIG. 15A illustrates a method for constructing a laser maskaccording to a first embodiment of the present invention.

Referring to FIG. 15A, a mask pattern 675A of the position ‘A’ is formedin a first block 680′ indicated by a square solid line, a mask pattern675C of the position ‘C’ is formed in a second block 680″, and a maskpattern 675B of the position ‘B’ is formed in a third block 680′″. Thethree mask patterns 675A to 675C are formed in the three blocks 680′ to680′″ of the laser mask according to the pattern constructing method ofthe present invention shown in FIG. 10 or FIG. 12. In the first block680′, twelve transmitting regions (the mask pattern 675A at the positionof ‘A’) having a circular shape are arranged in a 4 columns×3 rowsmatrix configuration. To be sure, the transmitting regions are arrangedcrisscross in the odd and even rows so that each row does not correspondwith each other, but it is assumed that the transmitting regions arearranged in the same row for convenience of explanation). Like the maskpattern 675A of the position ‘A’, total twelve transmitting regions ofthe mask pattern 675C are formed in a 4 columns×3 rows matrixconfiguration in the second block 680″. The positions of thetransmitting regions of the mask pattern 675C correspond to thepositions of the triangles in the first block 680′. Also, total 12transmitting regions of the mask pattern 675B of the position ‘B’ areformed in a 4 columns×3 rows matrix configuration in the third block680′″. The positions of the transmitting regions of the mask pattern675B correspond to the positions of the small squares in the first block680′.

In this manner, the position of each mask pattern 675A to 675C isconsistent with the pattern constructing method of the presentinvention. That is, assuming that the three patterns are formed in oneblock, on the basis of the transmitting regions positioned at the firstcolumn×first row of the mask pattern 675C (hereinafter ‘referencepattern’), the transmitting regions positioned at the first column×firstrow of the mask pattern 675A of the position ‘A’ is shifted by onecolumn leftward (that is, it is moved by a distance equal to one side ofthe small equilateral triangle indicated by a dotted line), and thetransmitting regions positioned at the first column×first row of themask pattern 675B of the position ‘B’ is shifted by one columnrightward. Except for the difference above, the three mask patterns 675Ato 675C formed in the three blocks 680′ to 680′″ have the sameconfiguration of 4 columns×3 rows.

In FIG. 15A, the mask patterns 675A to 675C are also formed outside thethree blocks 680′ to 680′″ indicated by the solid line. The three blocks680′ to 680′″ are virtual regions for constructing the periodic patterns675A to 675C on a laser mask, and thus the mask patterns 675A to 675Ccan be rearranged in accordance with process conditions such as a laserequipment and an optical system.

The blocks 680′ to 680′″ can be used as a reference for the next shotduring the three-shot crystallization method. According to this, amoving distance in the direction of X axis (namely, the X-step distance(Dx)) is substantially the same as the length of the horizontal side ofthe square (one block), and a moving distance in the direction of Y axis(namely, Y-step distance (Dy)) is substantially the same as the lengthof the vertical side of the square. The X-step distance (Dx) means amoving distance of the laser mask or the stage in the direction of Xaxis for the three-shot method, and the Y-step distance (Dy) means amoving distance of the laser mask or the stage in the direction of Yaxis to proceed with the Y-axis crystallization after the X-axiscrystallization. The Y-step distance (Dy) also means a moving distanceof the mask or the stage in the Y-axis direction so that a lower regionof the silicon thin film, which is not irradiated by the three-shotlaser beam during the X-axis crystallization, can be crystallized by thethree-shot method. The X-step distance (Dx) and the Y-step distance (Dy)are determined in consideration of the periodicity of the three blocks680′ to 680′″ in order to remove an X-overlap or a Y-overlap.

A laser mask having the three mask patterns described above will now bedescribed with an example. FIG. 15B illustrates an example of a lasermask fabricated by the pattern constructing method described withreference to FIG. 15A.

As described above, a laser mask 670 formed by the mask constructingmethod in accordance with the first embodiment of the present inventionhas three blocks with the mask pattern 675A of the position ‘A’, themask pattern 675C of the position ‘C’ and the mask pattern 675B of theposition ‘B’. The laser mask 670 blocks a laser beam except for thetransmitting regions of the mask patterns 675A to 675C formed with acertain periodicity. The mask 670 can be made of metal that can blocklight, such as chrome, aluminum, or the like. Although twelvetransmitting regions are formed in each block of the laser mask 670,more than twelve transmitting regions can be formed in each block inconsideration of process conditions such as laser equipment or opticalsystem.

A process for crystallizing a large-size silicon thin film using thelaser mask will now be described. FIGS. 16A to 16H illustrates asequential process for crystallizing a silicon thin film using the lasermask shown FIG. 15B.

As shown, three blocks are indicated by a solid line for convenience ofexplanation. Thus, each block is indicated by a square solid line. Inthis example, starting from left, a first block 680′ corresponds to themask pattern of the position ‘A’, a second block 680″ corresponds to themask pattern of the position ‘C’, and a third block 680′″ corresponds tothe mask pattern of the position ‘B’.

Referring to FIG. 16A, a first crystallization is performed byirradiating a laser beam onto a silicon film deposited on a substratethrough the laser mask shown in FIG. 15B. At this time, the laser beamhas an energy density corresponding to the complete melting region asdescribed in the earlier section, and crystals grow toward the centersof the circles using the amorphous silicon (solid phase) positioned atthe boundary surface as a nucleus, thereby forming polycrystalline firstcrystals 612′ in a first irradiated region (P1). The first crystals 612′have radial grains. In this case, the entirety of the first irradiatedregion (P1) is not crystallized, but a plurality of first crystals 612′having a circular shape are formed according to the pattern on the mask670. In detail, the regions crystallized by the first crystallizationcorrespond to the transmitting regions of the mask 670. Thus, if themask having the three blocks 680′ to 680′″ has thirty six transmittingregions, then the silicon film also has thirty six polycrystallinesilicon crystals 612′, each crystal having a certain radius.

After the first crystallization is completed, the stage (not shown) onwhich the substrate is placed or the mask 670 is moved in the X-axisdirection by a distance of the length of the side of the square (oneblock), which is the same as the X-step distance (Dx), and then a secondlaser beam is irradiated. The stage is moved by the X-step distance (Dx)in the “−X”-axis direction, for example, the first crystals 612′ of theposition ‘C’ formed by the second block overlaps the mask pattern of theposition ‘B’ in the third block, and then, a second laser beam isirradiated on the surface of the substrate. Then, as shown in FIG. 16B,the second crystals 612″ having the same shape as the first crystals612′ are formed. At this time, the positions of the second crystals 612″are shifted by the X-step distance (Dx) with respect to the firstcrystals 612′, and thus the second crystals 612″ overlap a portion ofthe first crystals 612′. The two center regions in FIG. 16B where thefirst laser shot and the second laser shot overlap each other, that is,where the first irradiated region (P1) and the second irradiated region(P2) overlap each other, are irradiated by the second-shot laser beam,so that starting from the circumferences of the first crystals 612′,crystals grow toward the centers of the pattern of the mask 670 of thesecond shot to form the polycrystalline second crystals 612″. In otherwords, the three first crystals 612′ which have been crystallizedthrough the first crystallization are positioned around the secondcrystals 612″, and the second crystallization starts from the region620′ where the mask pattern and the three first crystals 612′ overlapeach other, thereby forming the second crystals 612″ grown toward thecenters of the mask pattern (refer to FIG. 14B). In FIG. 16B, thecrystals formed by the first block of the mask 670 during the secondshot are the first crystals, not the second crystals.

Next, the stage or the mask 670 is moved again by the X-step distance(Dx) in the X-axis direction, and then a third laser beam is irradiatedto continuously proceed with the crystallization in the X-axisdirection. For example, after the second crystallization is performed bythe second laser shot, the third laser beam is irradiated on the surfaceof the substrate. At this time, the first crystals 612′ of the position‘A’ formed by the first block overlap the mask pattern of the position‘B’ in the third block. Then, as shown in FIG. 16C, the third crystals612′″ having the same shape as the first crystals 612′ are formed. Atthis time, the positions of the third crystals 612′″ are shifted by theX-step distance (Dx) with respect to the first crystals 612′, and thusoverlap a portion of the first crystals 612′ and the second crystals612″. At this time, the center region in FIG. 16C where the first,second and third laser shots overlap each other, that is, where thefirst irradiated region (P1), the second irradiated region (P2) and thethird irradiated region (P3) overlap each other (namely, the regioncorresponding to the third block of the mask 670 for the third laserirradiation), is irradiated by the third-shot laser beam, so thatstarting from the circumferences of the patterns of the second crystals612″, crystals grow toward the centers of the pattern of the mask 670 ofthe third shot to form the polycrystalline third crystals 612′″. Inother words, the three second crystals 612″ which have been crystallizedthrough the second crystallization are positioned around the thirdcrystals 612′″, and the third crystallization starts from the region620″ where the mask pattern according to the third shot overlaps thethree second crystals 612″, thereby forming the third crystals 612′″grown toward the centers of the mask pattern (refer to FIG. 14C). Inthis manner, after the three-shot crystallization is performed byapplying the laser mask having the three blocks, the three-shot regionis crystallized without an X-overlap, that is, without a shot mark, asshown in the drawing. That is, the region where the first crystals 612′,the second crystals 612″ and the third crystals 612′″ are all formedcorresponds to the three-short region which has been crystallizedwithout a shot mark. In FIG. 16C, the crystals formed by the secondblock of the mask 670 during the third shot are the second crystals 612″newly formed by the third laser shot after the first crystallization,not the third crystals, and the crystals formed by the first block ofthe mask 670 during the third shot are the first crystals 612′ newlyformed by the third laser shot, not the third crystals.

Next, as shown in FIG. 16D, the stage or the mask 670 further is movedagain by the X-step distance (Dx) in the X-axis direction, and then afourth laser beam is irradiated. Then, the three-shot crystal region (P)having uniform crystalline characteristics without an X-overlap or aY-overlap is formed in the center by the third-shot laser beam. Asdescribed above, the three-shot crystal region (P) corresponding to theregion of the third crystals 612′″ is formed without an X-overlap,namely, without a shot mark. Meanwhile, the crystallization process isrepeatedly performed in the X-axis direction. Then, as shown in FIGS.16E and 16F, the three-shot crystal region (P) without a shot markincreases in the X-axis direction. This three-shot crystal region (P) isa uniform crystal region without a shot mark that is formed using thelaser mask having three blocks, each block having a periodic pattern.

Meanwhile, the lower region of the silicon film is not completelyirradiated by a laser beam. This is because the crystallization processwas performed only in the X-axis direction. After the crystallizationprocess is completed in the X-axis direction (X-axis crystallization),in FIG. 16G the mask 670 or the stage is moved by the Y-step distance(Dy) in the Y-axis direction (in case of moving the stage, in thedirection of −Y axis), and then, the crystallization process describedabove with respect to the X-axis crystallization is continuouslyperformed in the direction of −X axis, starting from the end point wherethe first X-axis crystallization process was finished. In this case, thecrystallization is continuously performed by applying the same blocks ofthe mask 670 as the X-axis crystallization. The upper pattern (namely,the pattern formed beyond the block region) of the mask 670 ispositioned corresponding to the lower region which has not beencompletely irradiated by a laser beam after the first X-axiscrystallization. Thus, the lower region can be completely crystallizedby the crystallization process in the direction of the −X axis. Withthis procedure, the lower region crystallized by the crystallizationprocess in the direction of the −X axis can be formed without aY-overlap.

Thereafter, the above-described method is repeatedly applied in thedirections of X axis and Y axis to form an arbitrary crystallizedregion, as shown in FIG. 16H. In particular, the three-shot crystalregion (P) is a crystallized region without an X-overlap or a Y-overlapthat has uniform crystallization characteristics. This is because thisregion does not include a shot mark and the crystals have radial grains.

In this embodiment, a crystalline silicon thin film without an X-overlapor a Y-overlap is obtained by the three-shot method using the laser maskhaving three blocks, each block having a periodic pattern. Although themask patterns have the transmitting regions having a circular shape,they also can be formed to have a shape of a regular polygon, such asregular triangle, square, regular hexagon, regular octagon, or the like.In addition, although each block has twelve transmitting regions, thenumber of the transmitting regions in each block can be varied dependingon process conditions. Moreover, although the radius (R) of thetransmitting regions is one half of the distance (L) between the centersof the transmitting regions, it can be varied, as long as therelationship between R and L satisfies equation 1. In this embodiment,although the mask pattern of the position ‘A’, the mask pattern of theposition ‘C’ and the mask pattern of the position ‘B’ are positioned inorder in the first to the third blocks of the mask, the positions of themask patterns can be varied.

Another example of positioning the three mask patterns will now bedescribed in detail. FIG. 17 illustrates a method for constructingperiodic patterns in a laser mask according to a second embodiment ofthe present invention.

Referring to FIG. 17, in constructing circular transmitting regions (A,B and C) in the laser mask, the laser mask is divided into three blocksto remove a shot mark. A laser transmitting region 775A having theposition ‘A’ is formed in a first block, a transmitting region 775Bhaving the position ‘B’ is formed in the second block, and atransmitting region 775C having the position ‘C’ is formed in the thirdblock. That is, as mentioned above, the circular mask patterns 775A to775C are sequentially formed in each of the three blocks of the lasercrystallization mask. In this embodiment, the mask pattern 775B of theposition ‘B’ is positioned at the center of the regular hexagon shown inFIG. 17, but the present invention is not limited thereto. Thetransmitting region 775A of the position ‘A’ and the transmitting region775C of the position ‘C’ surround the reference point 775B. That is, themask pattern 775B of the position ‘B’ formed in the third block ispositioned at the center of the regular hexagon pattern, around whichthe different patterns (that is, the mask pattern 775A of the position‘A’ and the mask pattern 775C of the position ‘C’) are positioned.Meanwhile, the size and intervals of the mask patterns 775A to 775Cshould satisfy equation 1 in order to completely crystallize a siliconthin film by the three-shot method without a shot mark.

The laser mask having the mask patterns constructed by the above methodand a crystallization process using the laser mask will now bedescribed. FIG. 18A illustrates a method for constructing a laser maskin accordance with the second embodiment of the present invention. Thesecond embodiment of the present invention is the same as the firstembodiment of the present invention except for the order of positioningthe mask patterns of the position ‘B’ or ‘C’ in the second and thirdblocks.

Referring to FIG. 18A, a mask pattern 775A of the position ‘A’ ispositioned in a first block 780′ indicated by a square solid line, amask pattern 775B of the position ‘B’ is positioned in a second block780″, and a mask pattern 775C of the position ‘C’ is positioned in athird block 780′″. The three mask patterns 775A to 775C are formed inthe three blocks 780′ to 780′″ of the laser mask according to thepattern constructing method shown in FIG. 17. In the first block 780′,twelve transmitting regions having a circular shape (the mask pattern775A) are arranged in a 4 columns×3 rows matrix configuration. To besure, the transmitting regions are arranged crisscross in the odd andeven rows so that each row does not correspond with each other, but itis assumed that the transmitting regions are arranged in the same rowfor convenience of explanation). Like the mask pattern 775A of theposition ‘A’, total twelve transmitting regions of the mask pattern 775Bare formed in a 4 columns×3 rows matrix configuration in the secondblock 780″. The positions of the transmitting regions of the maskpattern 775B correspond to the positions of the small squares in thefirst block 780′ in FIG. 18A. Also, total twelve transmitting regions ofthe mask pattern 775C of the position ‘C’ is formed in a 4 columns×3rows matrix configuration. The positions of the transmitting regions ofthe mask pattern 775C correspond to the positions of the triangles inthe first block 780′ and in the third block 780′″ in FIG. 18A.

The position of each mask pattern 675A to 675C is consistent with thepattern constructing method of the present invention. That is, assumingthat the three patterns are formed in one block, on the basis of thetransmitting regions positioned at the first column x first row of themask pattern 775B (hereinafter ‘reference pattern’), the transmittingregions positioned at the first column×first row of the mask pattern775A are positioned at the vertexes of the left lower side of theequilateral triangle, and the transmitting regions positioned at the 1column×1 row of the mask pattern 775C are positioned at the vertexes ofthe right lower side of the equilateral triangle with respect to thereference pattern. The mask patterns 775A and the mask patterns 775C areshifted by a certain distance (one half of the length of one side of theequilateral triangle leftward/rightward, and the height of theequilateral triangle downward), the three mask patterns 775A to 775Cformed in the three blocks 780′ to 780′″ have the same configuration of4 columns×3 rows.

Compared to the laser mask constructing method according to the firstembodiment of the present invention, the laser mask constructing methodaccording to the second embodiment of the present invention has adifferent mask design. In other words, in the first embodiment, the maskpattern ‘C’ is positioned in the second block, whereas the mask pattern‘B’ is positioned in the second block in the second embodiment.

A laser mask formed by the pattern constructing method described abovewill now be described with an example. FIG. 18B illustrates an exampleof a laser mask fabricated by the pattern constructing method describedwith reference to FIG. 18A.

As described above, a laser mask 770 formed by the mask constructingmethod in accordance with the second embodiment of the present inventionhas three blocks with the mask pattern 775A of the position ‘A’, themask pattern 775B of the position ‘B’ and the mask pattern 775C of theposition ‘C’. In this embodiment, twelve transmitting regions are formedin each block of the laser mask 770, more than twelve transmittingregions can be formed in consideration of process conditions, such aslaser equipment or optical system, without being limited thereto.

A process for crystallizing a large-size silicon thin film using thelaser mask will now be described. FIGS. 19A to 19G illustrates asequential process for crystallizing a silicon thin film using the lasermask shown in FIG. 18B.

As shown, three blocks are indicated by a solid line for convenience ofexplanation. Thus, each block is indicated by a square solid line. Inthis example, starting from left, a first block 780′ corresponds to themask pattern of the position ‘A’, a second block 780″ corresponds to themask pattern of the position ‘B’, and a third block 780′″ corresponds tothe mask pattern of the position ‘C’.

Referring to FIG. 19A, a first crystallization is performed byirradiating a laser beam onto a silicon film deposited on a substratethrough the laser mask shown in FIG. 18B. At this time, the laser beamhas an energy density corresponding to the complete melting region asdescribed in the earlier section, and crystals grow toward the centersof the circles using the amorphous silicon (solid phase) positioned atthe boundary surface as a nucleus, thereby forming polycrystalline firstcrystals 712′ in a first irradiated region (P1). The first crystals 712′have radial grains.

After the first crystallization is completed, the stage (not shown) onwhich the substrate is placed or the mask 770 is moved in the X-axisdirection by a distance of the length (Dx) of the side of the square(one block), and then a second laser beam is irradiated. The stage ismoved by the X-step distance (Dx) in the “−X”-axis direction, forexample, the first crystals 712′ of the position ‘B’ in the second blockoverlap the mask pattern of the position ‘C’ in the third block, andthen, the second laser beam is irradiated on the surface of thesubstrate. Then, as shown in FIG. 19B, the second crystals 712″ havingthe same shape as the first crystals 712′ are formed. At this time, thepositions of the second crystals 712″ are shifted by the X-step distance(Dx) with respect to the first crystals 712′, and thus the secondcrystals 712″ overlap a portion of the first crystals 712′. The twocenter regions in FIG. 19B where the first laser shot and the secondlaser shot overlap each other, that is, where the first irradiatedregion (P1) and the second irradiated region (P2) overlap each other,are irradiated by the second-shot laser beam, so that starting from theregion 720′, where the mask pattern for the second shot overlaps thefirst crystals 712′, crystals grow toward the centers of the pattern ofthe mask 770 of the second shot to form the polycrystalline secondcrystals 712″ (refer to FIG. 14B). In FIG. 19B, the crystals formed bythe first block of the mask 770 during the second shot are the firstcrystals, not the second crystals.

Next, the stage or the mask 770 is moved again by the X-step distance(Dx) in the X-axis direction, and then a third laser beam is irradiatedto continuously proceed with the crystallization in the X-axisdirection. For example, after the second crystallization is performed bythe second laser shot, the third laser beam is irradiated on the surfaceof the substrate. At this time, the first crystals 712′ of the position‘A’ formed by the first block overlap the mask pattern of the position‘C’ in the third block. Then, as shown in FIG. 19C, the third crystals712′″ having the same shape as the first crystals 712′ are formed. Atthis time, the center region in FIG. 1 9C where the first, second andthird laser shots overlap each other, that is, where the firstirradiated region (P1), the second irradiated region (P2) and the thirdirradiated region (P3) overlap each other (namely, the regioncorresponding to the third block of the mask 770 for the third laserirradiation), is irradiated by the third-shot laser beam, so thatstarting from the circumferences of the patterns of second crystals 712″(specifically, the region 720″ where the mask pattern for the third shotoverlaps the second crystals 712″), crystals grow toward the centers ofthe pattern of the mask 770 of the third shot to form thepolycrystalline third crystals 712′″ (Refer to FIG. 14C). In thismanner, after the three-shot crystallization is performed by applyingthe mask having the three blocks, the three-shot region is crystallizedwithout an X-overlap, that is, without a shot mark as shown in thedrawing. In FIG. 19C, the crystals formed by the second block of themask 770 during the third shot are the second crystals 712″ newly formedby the third laser shot after the first crystallization, not the thirdcrystals, and the crystals formed by the first block of the mask 770during the third shot are the first crystals 712′ newly formed by thethird laser shot, not the third crystals.

Next, as shown in FIG. 19D, the state or the mask 770 is further movedagain by the X-step distance (Dx) in the X-axis direction, and then afourth laser beam is irradiated. Then, the three-short crystal region(P) having uniform crystalline characteristics without an X-overlap or aY-overlap is formed in the center by the third-shot laser beam. Asdescribed above, the three-shot crystal region (P) corresponding to theregion of the third crystals 712′″ is formed without an X-overlap, thatis, without a shot mark. Meanwhile, the crystallization process isrepeatedly performed in the X-axis direction. Then, as shown in FIGS.19E and 19F, the three-shot crystal region (P) without a shot markincreases in the X-axis direction. This three-shot crystal region (P) isa uniform crystal region without a shot mark that is formed using thelaser mask having the three blocks, each having a periodic pattern.

Next, after the crystallization process is completed in the X-axisdirection (first X-axis crystallization), the mask 770 or the stage ismoved by the Y-step distance (Dy) in the Y-axis direction (in case ofmoving the stage, in the direction of −Y axis), and then, thecrystallization process described above with respect to the X-axiscrystallization is continuously performed in the direction of −X axis,starting from the end point where the first X-axis crystallizationprocess was finished. In this case, the crystallization is continuouslyperformed by applying the same blocks of the mask 770 as the firstX-axis crystallization. The upper pattern (namely, the pattern formedbeyond the block region) of the mask 770 is positioned corresponding tothe lower region which has not been completely irradiated by a laserbeam after the first X-axis crystallization. The lower region can becompletely crystallized by the crystallization process in the directionof the −X axis. With this procedure, the lower region crystallized bythe crystallization process in the direction of the −X axis can beformed without a Y-overlap. Thereafter, the above-described method isrepeatedly applied in the directions of X axis and Y axis to form anarbitrary crystallized region, as shown in FIG. 19G.

In the second embodiment, although the mask pattern of the position ‘A’,the mask pattern of the position ‘C’ and the mask pattern of theposition ‘B’ are sequentially positioned in the first to the thirdblocks of the laser mask, it is not limited thereto. For example, eitherthe mask pattern of the position ‘C’ or the mask pattern of the position‘B’ can be positioned in the first block while maintaining the order ofthe three mask patterns in the same manner.

As described in the first and second embodiments of the presentinvention, laser beams are irradiated onto a silicon thin film throughthe three mask patterns formed in the three blocks of the laser mask.Accordingly, the portion of the silicon thin film irradiated by thethree-shot laser irradiation is completely crystallized. The three-shotmethod includes the first crystallization by the first shot, the secondcrystallization by the second shot and the third crystallization by thethird shot. In the first crystallization, crystals grow toward thecenters of the circles using the amorphous silicon thin film (solidphase) positioned around the circular pattern as a nucleus, that is atthe boundary surface of the circumferences. In the second and thirdcrystallizations, crystals grow toward the centers of the mask patternsof the second shot and the third shot using the circumferences of thefirst and second crystals as a start point.

A method for fabricating an LCD device using the silicon thin filmhaving the improved crystallization characteristics in accordance withthe present invention will now be described. FIG. 20 is a plan viewillustrating a structure of a liquid crystal display panel, in which adriving circuit is integrated with the array substrate of the LCD panel.

As shown, the driving circuit-integrated LCD panel includes an arraysubstrate 820, a color filter substrate 830, and a liquid crystal layer(not shown) formed between the array substrate 820 and the color filtersubstrate 830. The array substrate 820 includes a pixel unit 825, animage display region where unit pixels are arranged in a matrixconfiguration, and a gate driving circuit unit 824 and a data drivingcircuit unit 823 positioned at an outer edge of the pixel unit 825.Though not shown, the pixel unit 825 of the array substrate 820 includesa plurality of gate lines and data lines arranged vertically andhorizontally and defining a plurality of pixel regions on the substrate820. The pixel unit further includes a thin film transistor, a switchingdevice formed near the crossings of the gate lines and the data lines,and pixel electrodes formed at the pixel regions. As a switching devicefor applying a signal voltage to the pixel electrode, the thin filmtransistor (TFT) is a field effect transistor (FET) for controlling aflow of current by an electric field.

Of the array substrate 820, the data driving circuit unit 823 ispositioned at the longer side of the array substrate 820 which isprotruded compared with the color filter substrate 830, and the gatedriving circuit unit 824 is positioned at the shorter side of the arraysubstrate 820. In order to suitably output an inputted signal, the gatedriving circuit unit 824 and the data driving circuit unit 823 use athin film transistor with a CMOS (Complementary Metal OxideSemiconductor) structure, an inverter. For reference, the CMOS is anintegrated circuit with a MOS structure used for high signal processing,and needs P channel and N channel transistors. Its speed and densitycharacteristics are in between the NMOS and the PMOS. The gate drivingcircuit unit 824 and the data driving circuit unit 823, which aredevices for supplying a scan signal and a data signal to the pixelelectrode through the gate line and the data line, are connected to anexternal signal input terminal (not shown) so as to control an externalsignal transmitted through the external signal input terminal and outputit to the pixel electrode.

Though not shown, a color filter for implementing color and a commonelectrode, which is a counter electrode of the pixel electrode formed onthe array substrate 820, are formed on the image display region 825. Aspacer between the array substrate and the color filer substrate isformed to provide a uniform cell gap. The array substrate and the colorfilter substrate are attached by a seal pattern formed at an outer edgeof the image display region, to form a unit liquid crystal displaypanel. The two substrates are attached through an attachment key formedat the array substrate or the color filter substrate. The drivingcircuit-integrated LCD panel using the polycrystalline silicon thin filmhas many advantages in that it has excellent device characteristics,excellent picture quality, adequate display capability and low powerconsumption.

A driving circuit-integrated LCD device using the crystallized siliconthin film fabricated according to the present invention will now bedescribed through its fabrication process. FIG. 21 illustrates anexample of an LCD device fabricated using a silicon thin filmcrystallized by a crystallization method in accordance with the presentinvention. As for the thin film transistor (TFT) formed at the pixelunit, both N-type and P-type TFT are available. For the driving circuitunit, either the N-type TFT or the P-type TFT can be used as in thepixel unit, or the CMOS structure having both the N-type TFT and theP-type TFT can be also used. Herein, FIG. 21 illustrates an example ofthe CMOS liquid crystal display device.

A method for fabricating the CMOS LCD device will be described asfollows. First, a buffer layer 821 made of a silicon oxide film (SiO₂)is formed on a substrate 820 made of a transparent insulation materialsuch as glass. Next, active layers 824N and 824P made of polycrystallinesilicon are formed on the buffer layer-formed substrate 820. To thisend, after an amorphous thin film is formed on the entire surface of thesubstrate 820, the active layers 824N and 824P are sequentiallylaterally solidified by a three-shot crystallization method that is inaccord with the present invention. At this time, the three-shotcrystallization method uses a laser mask having three blocks, each blockhaving a periodic pattern. Accordingly, a uniform polycrystallinesilicon thin film can be formed without a short mark.

Thereafter, the crystallized polycrystalline silicon thin film ispatterned through a photolithography process in order to form the activepatterns 824N and 824P at the NMOS and PMO regions. Then, a gateinsulation film 825A is deposited on the active layers 824N and 824P.Subsequently, gate electrodes 850N and 850P made of molybdenum (Mo),aluminum (Al), an aluminum alloy or the like is formed on a certainregion (namely, a channel formation region of the active layers 824N and824P) on the gate insulation film 825. The gate electrodes 850N and 850Pare formed by a photolithography process after a gate metal is depositedon the gate insulation film 825A. Then, a N-doping process and aP-doping process are sequentially performed to form an N-type TFT(namely, a TFT having source/drain regions 822N and 823 formed byimplanting N+ ions at a certain region of the active layer 824N) and aP-type TFT. At this time, the source region 822N and the drain region823N of the N-type TFT are formed by injecting a fifth-group elementsuch as phosphor (P) that can donate an electron. The source/drainregions 822P and 823P of the P type TFT are formed by injecting athird-group element such as boron (B) that can donate a hole.Thereafter, an interlayer insulation film 825B is deposited on theentire surface of the substrate 820, and contact holes 860N and 860P areformed to expose a portion of the source/drain regions 822N, 822P, 823Nand 823P by a the photolithography process. Finally, source/drainelectrodes 851N, 851P, 852N and 852P are formed to be electricallyconnected with the source/drain regions 822N, 822P, 823N and 823Pthrough the contact holes 860N and 860P, thereby completing a CMOSliquid crystal display device. Although the present invention presents amethod for fabricating a LCD device having the crystallized silicon thinfilm, the principles of the present invention can be also applied toother display devices such as an organic EL, without being limitedthereto.

As so far described, the laser mask and the crystallization methodaccording to the present invention have many advantages. A laser maskaccording to the present invention has three blocks, each block havingits own periodic pattern. With a crystallization method according to thepresent invention, which uses the laser mask, a polycrystalline siliconthin film can be obtained without an X overlap or a Y overlap, that is,without a shot mark, by repeatedly applying the three blocks. Inaddition, by fabricating a liquid crystal display device using thepolycrystalline silicon thin film, the device can have uniform andimproved characteristics due to the crystallization characteristics ofthe active layer. Moreover, because the active layer does not have ashot mark, the picture quality of the liquid crystal display device canbe also improved.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the above-discussed displaydevice and the driving method thereof without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A laser mask having first, second and third blocks, each block having a periodic pattern including a plurality of transmitting regions and a blocking region, the periodic pattern of the first block having a first position, the periodic pattern of the second block having a second position, the periodic pattern of the third block having a third position, the first, second and third positions being different from each other.
 2. The laser mask according to claim 1, wherein when the three periodic patterns are projected in one block, the adjacent three transmitting regions form one equilateral triangle, and six of the equilateral triangles form a regular hexagon.
 3. The laser mask according to claim 1, wherein the transmitting regions have a shape of a circle.
 4. The laser mask according to claim 3, wherein a distance between the centers of the transmitting regions is L, a radius of the transmitting regions having a circular shape is R, and L and R have a relationship of $\frac{L}{3} \leq R < {\frac{L}{2}.}$
 5. The laser mask according to claim 1, wherein the transmitting regions have a shape of a polygon, the polygon including triangle, square, hexagon and octagon.
 6. The laser mask according to claim 1, wherein the transmitting regions in each block are arranged in an N columns×M rows matrix configuration (each of N and M is an integer).
 7. The laser mask according to claim 6, wherein the transmitting regions in each block are arranged crisscross in odd and even rows.
 8. The laser mask according to claim 1, wherein the laser mask is made of metal, the metal including chrome or aluminum.
 9. A crystallization method using a mask comprising: providing a substrate having a semiconductor layer; positioning a mask over the substrate, the mask having first, second and third blocks, each block having a periodic pattern including a plurality of transmitting regions and a blocking region, the periodic pattern of the first block having a first position, the periodic pattern of the second block having a second position, the periodic pattern of the third block having a third position, the first, second and third positions being different from each other; and crystallizing the semiconductor layer by irradiating a laser beam through the mask.
 10. The method according to claim 9, wherein when the three periodic patterns are projected in one block, the adjacent three transmitting regions form one equilateral triangle, and six of the equilateral triangles form a regular hexagon.
 11. The method according to claim 9, wherein the transmitting regions have a shape of a circle.
 12. The method according to claim 11, wherein a distance between the centers of the transmitting regions is L, a radius of the transmitting regions having a circular shape is R, and L and R have a relationship of $\frac{L}{3} \leq R < {\frac{L}{2}.}$
 13. The method according to claim 9, wherein the transmitting regions have a shape of a polygon, the polygon including triangle, square, hexagon and octagon.
 14. The method according to claim 9, wherein the transmitting regions in each block are arranged in an N columns×M rows matrix configuration (each of N and M is an integer).
 15. The method according to claim 14, wherein the transmitting regions in each block are arranged crisscross in odd and even rows.
 16. The method according to claim 9, wherein the mask is made of metal, the metal including chrome or aluminum.
 17. The method according to claim 9, wherein the crystallizing the semiconductor layer further includes: irradiating a first laser beam through the mask for forming a first crystallized region, wherein a size of the first crystallized region is W; moving the substrate by less than W; irradiating a second laser beam through the mask for forming a second crystallized region; moving the substrate by less than W; and irradiating a third laser beam through the mask for forming a third crystallized region.
 18. The method according to claim 17, wherein the substrate moves by about one third of W.
 19. The method according to claim 9, wherein the crystallizing the semiconductor layer further includes: irradiating a first laser beam through the mask for forming a first crystallized region; moving the mask by about a distance equal to a size of one block; irradiating a second laser beam through the mask for forming a second crystallized region; moving the mask by about a distance equal to a size of one block; and irradiating a third laser beam through the mask for forming a third crystallized region.
 20. The method according to claim 9, wherein the irradiated laser has an energy density of a complete melting region.
 21. The method according to claim 9, wherein the semiconductor layer is crystallized by a sequential lateral solidification (SLS) method.
 22. A display device comprising: a gate line and a data line crossing each other to form a pixel; a thin film transistor (TFT) near the crossing, the TFT including a polycrystalline silicon layer, wherein the polycrystalline silicon layer includes a plurality of circular crystals, and the three adjacent circular crystals form one equilateral triangle, and six of the equilateral triangles form a regular hexagon.
 23. A display device comprising: a gate line and a data line crossing each other to form a pixel; a thin film transistor (TFT) near the crossing, the TFT including a polycrystalline silicon layer, wherein the polycrystalline silicon layer includes a plurality of crystals having a polygon shape, and the centers of the three adjacent crystals form one equilateral triangle, and six of the equilateral triangles form a regular hexagon.
 24. A method for fabricating a display device comprising: forming a plurality of gate lines and data lines on a substrate, the gate and data lines crossing each other to define pixels; and forming a thin film transistor (TFT) near each crossing in the pixel, this step further including, forming a semiconductor layer on the substrate; positioning a mask over the substrate, the mask having first, second and third blocks, each block having a periodic pattern including a plurality of transmitting regions and a blocking region, the periodic pattern of the first block having a first position, the periodic pattern of the second block having a second position, the periodic pattern of the third block having a third position, the first, second and third positions being different from each other; crystallizing the semiconductor layer by irradiating a laser beam through the mask.
 25. The method according to claim 24, wherein when the three periodic patterns are projected in one block, the adjacent three transmitting regions form one equilateral triangle, and six of the equilateral triangles form a regular hexagon.
 26. The method according to claim 24, wherein the transmitting regions have a shape of a circle.
 27. The method according to claim 26, wherein a distance between the centers of the transmitting regions is L, a radius of the transmitting regions having a circular shape is R, and L and R have a relationship of $\frac{L}{3} \leq R < {\frac{L}{2}.}$
 28. The method according to claim 24, wherein the transmitting regions have a shape of a polygon, the polygon including triangle, square, hexagon and octagon.
 29. The method according to claim 24, wherein the transmitting regions in each block are arranged in an N columns×M rows matrix configuration (each of N and M is an integer).
 30. The method according to claim 29, wherein the transmitting regions in each block are arranged crisscross in odd and even rows.
 31. The method according to claim 24, wherein the mask is made of metal, the metal including chrome or aluminum.
 32. The method according to claim 24, wherein the crystallizing the semiconductor layer further includes: irradiating a first laser beam through the mask for forming a first crystallized region, wherein a size of the first crystallized region is W; moving the substrate by less than W; irradiating a second laser beam through the mask for forming a second crystallized region; moving the substrate by less than W; and irradiating a third laser beam through the mask for forming a third crystallized region.
 33. The method according to claim 32, wherein the substrate moves by about one third of W.
 34. The method according to claim 24, wherein the crystallizing the semiconductor layer further includes: irradiating a first laser beam through the mask for forming a first crystallized region; moving the mask by about a distance equal to a size of one block; irradiating a second laser beam through the mask for forming a second crystallized region; moving the mask by about a distance equal to a size of one block; and irradiating a third laser beam through the mask for forming a third crystallized region.
 35. The method according to claim 24, wherein the irradiated laser has an energy density of a complete melting region.
 36. The method according to claim 24, wherein the semiconductor layer is crystallized by a sequential lateral solidification (SLS) method.
 37. The device according to claim 22, wherein the display device is either a liquid crystal display or an electroluminescence display.
 38. The device according to claim 23, wherein the display device is either a liquid crystal display or an electroluminescence display.
 39. The method according to claim 24, wherein the display device is either a liquid crystal display or an electroluminescence display.
 40. The device according to claim 22, wherein each circular crystal includes a plurality of radially grown grains.
 41. The device according to claim 23, wherein each crystal includes a plurality of radially grown grains. 